Congenital abnormalities of the kidney and many disease processes affecting renal function are best understood in the context of kidney development. This proposal aims to study the role of canonical Wnt signaling in the novel context of the zebrafish pronephric kidney, and specifically the roles that canonical Wnt signaling and Frizzled receptors play in kidney differentiation and mesenchymal-epithelial transition (MET). The specific aims of this proposal will: 1) Define spatial and temporal requirements for canonical Wnt signaling in the zebrafish pronephros using chemical and inducible genetic methods. 2) Investigate the role of Frizzled4 and Frizzled9 in kidney development. 3) Assess the ability of cells persistently activating or inhibiting canonical Wnt signaing or expressing activated Frizzleds to participate in kidney differentiation and MET. Wnt signaling is known to be critical for mammalian metanephric kidney formation and epithelial differentiation but the receptors mediating this process are unknown. This study utilizes the structurally simpler zebrafish pronephros to study Frizzled receptors and the cell autonomous responses to Wnt signaling in kidney development and MET. Transgenic lines created will be of use to other researchers studying Wnt signaling in the fish. This work is likely to reveal mechanistic insights applicable to mammalian systems and MET in other organs.